Maitotoxin, a highly potent marine toxin,has the ability to modulate various calcium-dependent cellular processes by increasing calcium influx into the cells. Treatment of different types of cell types with maitotoxin resulted in their death as indicated by the leakage of the cytosolic enzyme lactate dehydrogenase(LDH). The TD50 of LDH leakage ranged from 60 - 800 pM when rat liver, Reuber hepatoma, rat glioma C6, and neuroblastoma-glioma NG 108-15 cells were incubated with maitotoxin for a period of 24 hours. The cell death was not observed when calcium was omitted from the medium indicating that the presence of extracellular calcium was essential for the toxic effect of maitotoxin. A massive influx of calcium(45Ca) was observed in hepatoma cells following treatment with the toxin. The calcium influx preceded cell death as measured by LDH leakage. The LDH leakage in all the cell types studied was preceded by the release of adenine nucleotides into the medium. This was measured by monitoring the leakage of radioactivity from cells prelabeled with [14C]adenine. This parameter which is a measure of ATP depletion exceeded LDH leakage. Thus. the maitotoxin- induced cell death appears to be the result of ATP depletion caused by the inhibition of oxidative phosphorylation produced by a large influx of calcium into cells. A rapid increase in free cytosolic calcium was elicited when Fura-2 loaded rat liver ARL-15 cells were incubated with the toxin. This increase was inhibited in a dose-dependent manner by verapamil, a well characterized calcium channel blocker. These results indicate that maitotoxin causes cell death in a variety of cell lines by depletion of cell ATP caused by massive influx of calcium.